Conventionally, various types of image forming apparatuses wherein a visible image is formed directly on a recording material such as a sheet, etc., by flying toner (developing material) based on an image signal have been proposed. The described image forming apparatuses include at least a toner holding member for holding toner in one color, a counter electrode placed opposing the toner holding member, and a control electrode having a plurality of opening sections which serve as toner passing holes, the control electrode being placed between the toner holding member and the counter electrode. These apparatuses are arranged so as to control a flying of the toner by varying a potential to be applied to the control electrode.
After the described image forming apparatuses have been used for a long period of time, such problem that the toner adheres to the opening sections of the control electrode, or irregularities in the resulting image would occur. In order to eliminate the described problems, it is required to clean up the control electrode periodically by taking it out from the apparatus main body, or exchange the control electrode with new control electrode.
For example, Japanese Unexamined Patent Publication No. 336049/1994 (Tokukaihei 6-336049) discloses an image forming apparatus wherein a control electrode and a toner holding member are mounted to the first flame which is mounted to the apparatus main body, and a counter electrode is mounted to the second flame which is axially supported by the apparatus main body so as to be rotatable relative to the first flame.
According to the described arrangement, when rotating the second flame relative to the first flame, by making these flames apart from one another, the counter electrode and the control electrode apart from one another. As a result, the control electrode is exposed, and a cleaning up of the control electrode can be performed with ease.
Additionally, in the conventional image forming apparatus, when a damage occurs in some members of the apparatus, or the life of some members are ended, and a removal and exchange of these members are required, as the members of the apparatus are provided independently, a long time is required for the removal or the exchange operation, and such operation cannot be performed efficiently.
In order to counteract the described problems, Japanese Unexamined Patent Publication No. 67961/1992 (Tokukaihei 4-67961) teaches that at least two of the toner holding member, the counter electrode and the control electrode are formed in a unit. According to the described arrangement, when a member within the unit is damaged, or its life is ended, and an exchange of such fault member is required, as such member can be exchanged in a unit, the process for the exchange operation of the fault member can be simplified, and the burden of the user can be reduced.
For example, Japanese Unexamined Patent Publication No. 286166/1993 (Tokukaihei 5-286166) discloses an arrangement wherein toner supply means with a toner holding member and a control electrode are integrated, so as to permit both to be removed at the same time. The structure of this Publication also permits the control electrode to be removed independently. According to the described arrangement, only the control electrode can be exchanged without taking out the toner supply means from the apparatus main body, and the toner supply means can be used to the end of its life. This can eliminate such problem that the member which can be still used has to be exchanged together with the fault member or the member which has been used to the end of its life can be eliminated.
Japanese Unexamined Patent Publication No. 305180/1994 (Tokukaihei 6-305180) discloses a recording device in which the control electrode and the counter electrode are integrated. According to the described arrangement, a constant interval is maintained between the control electrode and the counter electrode.
Specifically, in the case where the toner supply means and the control electrode are integrated, when exchanging the unit for a supply of the toner, it is likely that a small interval (for example, 0.5 mm) formed between the control electrode and the counter electrode varies. As a result, a path for the toner which passes through the opening sections of the control electrode varies, which causes irregularities of the toner image. Especially, when the interval becomes smaller than the predetermined distance, an insulating brakedown would occur between the control electrode and the counter electrode, which may cause the problem that a recording operation can no longer be performed by the apparatus.
Therefore, according to the described image forming apparatus, as the interval between the control electrode and the counter electrode can be maintained always constant, a flying of the toner can be controlled under stable conditions, thereby permitting an image forming operation to be performed under stable conditions.
The structure which offers a constant interval between the control electrode and the counter electrode is also disclosed by Japanese Unexamined Patent Publication No. 255161/1994 (Tokukaihei 6-255161). Namely, according to the recording device of this Publication, by providing the spacer between the portion outside the recording region of the control electrode and the counter electrode, the interval can be maintained constant to control a flying of a toner under stable conditions.
For example, Japanese Unexamined Patent Publication No. 108566/1996 (Tokukaihei 8-108566) discloses a printer unit wherein at least two toner holding members and the control electrode divided in a number of the toner holding members are formed in a unit. According to the described arrangement, by providing the divided control electrodes corresponding to the respective toner holding members, a reduction in amount of the toner passing through the opening section of the control electrode can be prevented. As a result, such problem that a bright line appears in an image, i.e., a so-called white line noise can be eliminated.
Additionally, in the printer unit, by reducing an area for each control electrode by dividing the control electrode, such problem that a concentration of a black color varies especially when printing an image of a large area, i.e., a so-called a curtain effect can be eliminated.
However, according to the described arrangement of Japanese Unexamined Patent Publication No. 336049/1994 (Tokukaihei 6-336049), for example, when removing a jammed sheet, exchanging the toner cartridge, or performing various checks for printing inferior by opening the second flame provided so as to be capable of opening and closing, the control electrode is always exposed. Therefore, when a finger of the operator, etc., touches the control electrode, the surface of the control electrode is soiled or damaged.
Additionally, in each of the described conventional arrangements, for example, when a unit including, for example, the control electrode is removed from the apparatus, as the surface of the control electrode is not protected, the control electrode may be damaged due to an contact with other member.
As described, according to the conventional arrangement, a flying of the toner cannot be controlled desirably due to a damage of the control electrode. As a result, desirable printing performances cannot be maintained.